Why should the leaders of countries today commit their
governments and their people to the hard work and expense of a
national programme of soil conservation?

The answer is that soil takes many years to create, but it can
be destroyed in almost no time at all. With the loss of soil goes
man's ability to grow food crops and graze animals, to produce
fibre and forests. It is not enough to describe the soil as a
country's greatest source of wealth; it is more than that; it is
a country's life. And in one country after another today, the
soil is washing or blowing away.

Soil covers most of the land surface of the earth in a thin
layer, ranging from a few centimetres to several metres deep. It
is composed of rock and mineral particles of many sizes mixed
with water, air, and living things, both plant and animal, and
their remains.

As man measures time, soil formation is extremely slow. Where
the climate is moist and warm, it takes thousands of years to
form just a few centimetres of soil. In cold or dry climates, it
takes even longer, or soil may not form at all. While soil is
technically a renewable resource, its slow rate of formation
makes it practically irreplaceable.

Soil is a dynamic mixture, forever changing as water comes and
goes and plants and animals live and die. Wind, water, ice, and
gravity move soil particles about, sometimes slowly, sometimes
rapidly. But even though a soil changes, the layers of soil stay
much the same during one human lifetime unless they are moved or
scraped, or ploughed by man.

All soil is full of life, and good soils are teeming with it.
Plants and animals help keep the soil fertile. Plant roots tunnel
through the soil and break it up, and decaying plants form humus.
Burrowing animals mix the soil; the excrete of animals contribute
nutrients and improve soil structure.

Besides the soil's more obvious inhabitants, which include
rodents, insects, mites, slugs and snails, spiders, and
earthworms, there are countless microscopic residents, some
helpful to man and his crops, some harmful.

Good soils seem to hold the greatest populations of bacteria.
Almost without exception, bacteria are involved in basic enzyme
transformations that make possible the growth of higher plants,
including our food crops. From man's point of view, bacteria may
well be the most valuable of the life forms in soil.

Chemical reactions occur in the soil as a result of exchange
of positive ions, or cations. More exchanges take place in clay
soils than in any other type. These chemical reactions are also
essential to plant growth and development and are a good index of
soil fertility.

Man's chief interest in soil is for agriculture, but not all
soils are suitable for farming. The total land area of the world
exceeds 13 billion hectares, but less than half can be used for
agriculture, including grazing. A much smaller fraction - about
1.4 billion hectares - is presently suitable for growing crops.
The rest of the land is either too wet or too dry, too shallow or
too rocky. Some is toxic or deficient in the nutrients that
plants require and some is permanently frozen .

Europe, Central America, and North America have the highest
proportion of soils suitable for farming, although a number of
the more developed countries seem intent on paving over much of
their best farmland with roads and buildings. The lowest
proportions of arable soils are in North and Central Asia, South
America, and Australia. The single most serious drawback to
farming additional land is lack of water.

Civilizations began where farming was most productive. When
farm productivity declined, usually as a result of soil
mismanagement, civilizations also declined - and occasionally
vanished entirely.

Of the three requisites for a thriving civilization: fertile
soil, a dependable water supply and relatively level land with
reasonable rainfall which would not cause erosion, it is likely
that the third factor was most important, and evidence is
mounting that soil degradation has toppled civilizations as
surely as military conquest. In countries bordering the
Mediterranean, deforestation of slopes and the erosion that
followed has created man-made deserts of once productive land.
Ancient Romans ate well on produce from North African regions
that are desert today.

A recent study of the collapse in Guatemala around 900 AD of
the 1700 year-old Mayan civilization suggests that it fell apart
for similar reasons. Researchers have found evidence that
population growth among the Mayans was followed by cutting trees
on mountainsides to expand areas for farming. The soil erosion
that resulted from growing crops on steeper and steeper slopes
lowered soil productivity - both in the hills and in the valleys
- to a point where the populations could no longer survive in
that area. Today only empty ruins remain.

The same process of soil degradation which destroyed
civilizations in the past are still at work today.

Firstly, billions of tons of soil are being physically lost
each year through accelerated erosion from the action of water
and wind and by undesirable changes in soil structure.

Secondly, many soils are being degraded by increases in their
salt content, by waterlogging, or by pollution through the
indiscriminate application of chemical and industrial wastes.

Thirdly, many soils are losing the minerals and organic matter
that make them fertile, and in most cases, these materials are
not being replaced nearly as fast as they are being depleted.

Finally, millions of hectares of good farmland are being lost
each year to nonfarm purposes; they are being flooded for
reservoirs or paved over for highways, airports, and parking
lots. The result of all this mismanagement will be less
productive agricultural land at a time when world population is
growing and expectations are rising among people everywhere for a
better life.

The most serious form of soil degradation is from accelerated
erosion. Erosion is the washing or blowing away of surface soil,
sometimes down to bedrock. While some erosion takes place without
the influence of man, the soil is lost so slowly that it is
usually replaced through natural processes of decay and
regeneration. Soil loss and soil creation of new soil stay in
balance.

What keeps soil in a natural state from eroding is vegetation.
Undisturbed by man, soil is usually covered by a canopy of shrubs
and trees, by dead and decaying leaves or by a thick mat of
grass. Whatever the vegetation, it protects the soil when the
rain falls or the wind blows. The leaves and branches of trees
and the cushion of grass absorb the force of raindrops, and root
systems of plants hold the soil together. Even in drought, the
roots of native grasses, which extend several metres into the
ground, help tie down the soil and keep it from blowing away.

With its covering of vegetation stripped away, however, soil
is as vulnerable to damage as a tortoise without its shell.
Whether the plant cover is disturbed by cultivation, grazing,
burning, or bulldozing, once the soil is laid bare to the erosive
action of wind and water, the slow rate of natural erosion is
greatly accelerated. Losses of soil take place much faster than
new soil can be created, and a kind of deficit spending begins
with the topsoil.

Unfortunately, many bad farming and forestry operations
encourage erosion. Erosion accelerates when sloping land is
ploughed and when grass is removed from semi-arid land to begin
dryland farming. It accelerates when cattle, sheep and goats are
allowed to overgraze and when hillside forests are felled or cut
indiscriminately. While there are isolated instances of deserts
being reclaimed by irrigation or of new forests being planted,
man, in the majority of instances, degrades the soil when he
begins agricultural operations.

And his highest risk operations are conducted on cropland,
which is particularly prone to the hazards of soil erosion,
especially if farming systems leave the land bare for part of the
year, exposed to wind and water.

The mechanics of soil erosion are fairly well understood today
by conservationists and by many farmers. Erosion from water
proceeds in three steps: (1) soil particles are loosened by the
bomb-like impact of raindrops or the scouring action of runoff
water; (2 ) the detached particles are moved down the slopes by
flowing water; and (3) the soil particles are deposited at new
locations, either on top of other soil at the bottom of the slope
or in ponds or waterways. The soil washed downhill is usually the
most fertile, containing most of the nutrients and organic matter
required for normal plant growth.

All other things being equal, the steeper the slope, the
greater the soil erosion. Erosion is also more severe on long
slopes than on short ones; the velocity of the water flow
increases on long, unobstructed downhill stretches. Soil loss may
be half again as great when the slope length is doubled.

Also significant is the shape of the slope. A convex or
bulging slope loses more soil than a uniform slope. A concave or
dish-shaped slope loses less. Many erodible soils also seal off
the surface pores of the soil as they travel downhill with the
runoff water. This action further decreases the amount of water
that can be absorbed by the soil and increases the water's
velocity, causing even more erosion.

Still another factor in soil erosion from water is the
erosivity of the rain its intensity and duration. In many parts
of Europe, where rains are relatively gentle, erosion is rarely
severe. In most tropical countries and in parts of the United
States, however, rains are much more intense and occasionally
torrential. Much more rain falls per hour, and as rainfall
intensity increases, the size of individual raindrops also
increases. A tropical raindrop strikes unprotected soil with more
force than raindrops in Europe, dislodging more soil. The flow of
water down a slope is also greater, and the net result is more
soil eroded and moved downhill.

Time is also a factor in erosion. A hard continuous rain will
dislodge more soil than several brief showers, particularly when
the soils are relatively impermeable.

Season is a factor, too. The monsoon rain in the Indian
subcontinent keeps farmers from planting many soils, and the bare
fields are subject to serious soil erosion. In the Corn Belt of
the USA, spring rains are usually the heaviest of the year,
striking the soil before seed can be planted or when seedlings
can be easily washed out.

Another factor in water erosion is the character of the soil
itself. Some soils tend to erode easily from the action of rain
and runoff; others are remarkably resistant, even in heavy
downpours. The susceptibility of different kinds of soils to
erosion under cultivation varies widely. Perhaps the most
important factor is the relative ability of the soil to absorb
rainfall rapidly. Certain soils of the tropics absorb rainfall so
rapidly that there is little erosion, even on steep slopes.

On the other hand, some erodible tropical soils require very
little energy to disintegrate under the impact of raindrops. One
reason for the instability of many tropical soils is the
predominance of coarse particles, which are easily detached by
the pounding action of the rain. The finer particles are then
washed off the field with the runoff water.

A number of the world's most erodible soils have a topsoil
layer that is from 10 to 40 centimetres deep, underlain by a
layer of subsoil that is barely permeable by water. After the
upper layer of soil becomes saturated by rain, it begins to flow
downhill, even on gentle slopes.

What makes one soil subject to erosion and another relatively
impervious is a complex matter. There is no single cause for
erodibility. But without question, the organic matter in the soil
- decayed and decaying plant and animal matter - helps protect it
from washing.

Organic matter in soil can absorb and store much more water
than can inorganic fractions. It acts like a sponge, taking up
water and releasing it as required by plants. It also helps bind
soil particles into larger aggregates, or crumbs. Soils with this
kind of structure are very resistant to erosion. Conversely,
nearly all soils containing little or no organic matter are very
susceptible to erosion.

Besides absorbing water readily, a good cropland soil should
be able to dry out or warm up quickly when the rain is over. It
should hold enough moisture to supply the needs of a crop between
rains, yet permit water to pass through the soil. A good soil
will not stay too wet or too dry.

Another factor in erosion from water is the crop that is being
grown in the soil and the way that crop is being managed. Sloping
land planted with trees or grass will erode less than the same
land planted with maize or soybeans. Maize planted on terraces
will suffer less erosion than maize planted in rows that march
straight down the slope, inviting runoff water to rush downhill
between the rows.

There are other, less obvious relationships between soil
erosion and crop selection and management. Many soils can be
planted with maize without much erosion risk if the maize crop is
rotated with legumes and small grains. If maize is planted year
after year, however, soil losses begin to mount.

The basic factors then that contribute to soil erosion from
water in rainfed agriculture appear to be similar the world over.
For any particular plot of land, they include the degree of
slope, the length of slope and its shape, the erosivity of the
rain and inherent erodibility of the soil, and the mismanagement
of the land by the farmer or herdsman. Much more remains to be
learned, however, about the management of specific soils in
tropical and subtropical areas to reduce the impact of these
erosion factors.

There are several types of man-made erosion, all but the first
clearly recognizable as trouble. The first - and most insidious -
is sheet erosion, which is the more or less even removal of a
thin layer or "sheet" of soil from a sloping field. It
is insidious because the amount of soil seen to be removed is
usually so small in any given year that a farmer often fails to
notice that erosion is occurring. Occasionally he becomes aware
of sheet erosion only after he notices that a formerly buried
object - a rock, the lower portion of a fence post, or root of a
tree - is suddenly exposed.

However, sheet erosion removes great quantities of topsoil.
Even a very thin layer of soil, only slightly thicker than a
piece of wrapping paper, when transported down a slope, can weigh
several tons per hectare. It does not take many years or many
rainstorms for losses from sheet erosion to become significant.

A second variety of erosion is more evident to the farmer, and
that is "rill" erosion. Sheet erosion occurs mainly
when the surface of a field is smooth and the slope is uniform.
But the surface of most fields is irregular. There are apt to be
low places and high places; rough places and smooth places; and
various kinds of soils, even in a 5 hectare field. When it rains,
the soil erodes unevenly, and rainwater accumulates and flows
into depressions, taking the path of least resistance as it moves
downhill. The surface flow moves into small channels, or rills,
which are cut into the soil several inches deep. Rills are small
enough to be erased easily with normal tillage methods, but left
alone, they can become progressively wider and deeper until they
cut into the subsoil and form gullies.

A gully always begins at the lower end of a slope and eats its
way back uphill, where it creates a gully head with a sudden or
steep fall. Eventually it will work its way to the top of the
slope, growing deeper and wider with each rainstorm. The splash
action of the falling water at the head of the gully undermines
the lower part of the excavated earth wall, causing collapse of
even more of the soil.

Unlike a rill, a gully cannot be smoothed out with a plough or
a disk. While a new gully may be narrow and 2 or 3 feet deep,
older gullies can grow to enormous size - 40 feet deep and as
much as 100 feet wide.

The formation of gullies is frequently encouraged by man and
his animals. Many gullies begin with stock trails, farm roads,
and other regular or irregular pathways on sloping land. Some
large gullies develop tributaries, particularly at points where
livestock habitually enter and leave a ravine.

A recent study of the development in the 19th century of
severe gully erosion at the head of a creek in New South Wales,
Australia, revealed that it began during periods of cultivation
and overgrazing and, not incidentally, during the years of the
highest rabbit populations. These animals, like many insects, can
speed up destruction of vegetation and soil erosion.

Gullies are relentless destroyers of good farmland. They can
cut up a field into small, odd-shaped parcels and restrict the
free movement of animals and farm machinery. They are a menace to
livestock; calves and other animals frequently fall in and are
unable to escape. Gullies can also threaten nearby barns and
other buildings, which may have to be moved before they are
undermined.

The stabilization and repair of gullies is the most costly of
all erosion control work. Stopping a gully often requires
extensive earthmoving and construction of dams or other measures.
On the other hand, the formation of gullies can usually be
prevented through good land use.

For the farmer, and for the consumer as well, the worst thing
about soil erosion is that it reduces crop yields and increases
the costs of growing food and fibre.

Firstly, erosion reduces the capacity of the soil to hold
water and make that water available to plants. This subjects
crops to more frequent and severe water stress.

Secondly, erosion contributes to losses of plant nutrients,
which wash away with the soil particles. Because subsoils
generally contain fewer nutrients than topsoils, more fertilizer
is needed to maintain crop yields. This, in turn, increases
production costs. Moreover, the addition of fertilizer alone
cannot compensate for all the nutrients lost when topsoil erodes.

Thirdly, erosion reduces yields by degrading soil structure,
increasing soil erodibility, surface sealing and crusting. Water
infiltration is reduced, and seedlings have a harder time
breaking through the soil crust.

Fourthly, erosion reduces productivity because it does not
remove topsoil uniformly over the surface of a field. Typically,
parts of an eroded field still have several inches of topsoil
left; other parts may be eroded down to the subsoil. This makes
it practically impossible for a farmer to manage the field
properly, to apply fertilizers and chemicals uniformly and obtain
uniform results. He is also unable to time his planting, since an
eroded part of the field may be too wet when the rest of the
field is dry and ready.

Damage from water erosion is not limited to the loss of
productivity on the land where it occurs. The bulk of eroded soil
from a hillside comes to rest a short distance away, at the foot
of the slope or on a nearby flood plain, where it may bury crops
or lower the fertility of bottomlands. A portion of the eroded
soil is deposited in local drainage or irrigation ditches or runs
into ponds, reservoirs, or tributary streams and rivers. Wherever
it is deposited, it is unwelcome. Sediment-filled ditches have to
be dug out again; ponds, lakes, and reservoirs either have to be
dredged out or abandoned. Locally, sediment is an expensive
nuisance .

Damage also occurs downstream, sometimes at great distances
from the farmland that originally contributed the sediment.
Carried along by a river, sediment is dropped out as the waterway
reaches flatter, lower reaches. The sediment deposits raise the
level of the riverbed and reduce the capacity of the channel to
hold water. Riverbanks overtop more frequently, and valuable
bottomland, often extremely productive, is damaged by flooding.

Soil blown by wind is second only to erosion by water as a
destroyer of agricultural land. It occurs most often in arid and
semi-arid regions, but it can also happen in areas of seasonal
rainfall. Wind erosion is a persistent hazard in the Sahara and
Kalahari deserts of Africa; in Central Asia, particularly in the
Steppes of the Soviet Union; in central Australia, and in the
Great Plains of the United States, well known as the Dust Bowl of
the 1930s.

Windborne topsoil may be transported over very long distances
and, like soil eroded by water, it is usually deposited where it
is not wanted.
Farmlands, fences, machinery, and buildings can be severely
damaged by wind erosion, and sometimes they can be buried
completely. Costs of rehabilitation can run so high that the land
is abandoned.

The following conditions set the stage for erosion from wind:

the soil is loose, dry, and finely divided;

the soil surface is relatively smooth and plant cover is
sparse; and

the field is sufficiently large and the wind strong
enough to initiate air movement.

When the wind blows hard over a smooth field, at some point
near the surface the wind velocity will be zero. Above that point
there is a layer of smooth airflow, and above that, an area of
turbulence. It is this turbulent airflow which causes soil
particles to begin to move. Once movement is begun, the soil
particles themselves abrade the soil surface and magnify the
effect of the wind. In a severe storm, dust clouds rise hundreds
of metres into the air, and on occasion travel hundreds, even
thousands, of kilometres before the eroded soil falls on the land
or into the ocean.

The soil particles that are blown away are usually the finer
ones; the coarse and heavy sand remains. If this process
continues for long, the productivity of the damaged land
gradually decreases.

The physical causes of wind erosion are clearly different from
those which allow soil to wash, except for one factor that is
constant in all man-made soil erosion - the absence of vegetation
to hold and cover the soil. It is when trees, bushes, grasses,
and other plants are removed from land that erosion occurs.

Soil does not have to be washed or blown away for its
productivity to be lowered. Through improper soil and water
management, a soil's properties may be altered so that its
fertility is seriously reduced or lost for good. Excessive
cultivation, for example, can wreck the structure of some soils
so that they are no longer capable of holding enough moisture for
growing plants.

Salinization, or the accumulation of salts in the topsoil, can
also have a deletrious effect on soil productivity and crop
yields. In extreme cases, damage from salinization is so great
that it is technically unfeasible or totally uneconomic to
reverse the process.

In general, salinization is caused by water and dissolved
salts moving up in the soil through capillary action. While
salinization is occasionally the result of natural soil-forming
processes, it occurs most frequently in irrigated soils, where it
is worsened by the high salt content of irrigation water.

Salt-affected soils are found on every continent and nearly 7
percent of the land area of the world is affected. Salinization
is a serious problem in Australia, the Soviet Union, and the
United States, and it is critical in countries of north Africa
and the Near East.

Waterlogged soils also deter agriculture in many countries,
even in parts of the world where an excess of water is not
usually thought of as a problem. Waterlogging interferes with
agriculture in many countries; in Egypt, for example, where about
one-third of the Nile Delta has a water table only 80 centimetres
below the surface. Other countries with waterlogging from high
water tables and runoff include Iran, Iraq, Somalia, parts of
Syria, and Pakistan.

Soil can also become degraded through loss of nutrients -
chiefly nitrogen, phosphorus, and potassium - if these are not
replenished to maintain soil fertility. Besides being lost
through erosion, nutrients are also depleted by the crops
themselves, particularly if the same crops are grown on the same
land year after year. And in the humid tropics, many nutrients
are leached during the intense rainstorms, especially on
unprotected land. Without question, farming all over the world is
removing more nutrients from the soil than are being put back.

Soil compaction is still another destroyer of the soil.
Sometimes it results from repeated passes over the same field
with heavy machinery, particularly when the field is wet. It can
also result from the hooves of grazing animals pounding down the
soil too often in the same area, as they do around the only
waterhole for miles. Compaction is not easy to correct.

Other forms of soil degradation occur in the more developed
countries, but are rarely of concern to the developing ones - so
far. Farmland is not only paved over by urbanization but is
occasionally poisoned with chemicals. While pesticides and even
fertilizers are sometimes suspected of causing soil impairment,
the damage in most cases is not permanent. However, some apple
orchards sprayed with arsenic compounds in the 1930s were
reported as still unproductive 30 years later. In recent years,
there has been a general movement in many developed countries
against using the more persistent insecticides, including a
chemical group that includes DDT and chlordane. Radioactive
fallout, and Strontium 90 in particular, also caused public
concern during the period of nuclear bomb tests.

Today a more serious problem in several highly industrialized
countries is the indiscriminate dumping of chemical wastes, some
of which are extremely toxic to plants, animals, and man, and the
growing use of sewage sludge, some of which contains dangerous
heavy metals which can be taken up by plants. For a developing
nation, however, such problems are at present insignificant
compared with the growing threat to their agricultural
productivity from erosion, salinization, waterlogging, and
general loss of fertility.